Field of the Invention
The present invention relates to a galvanized steel sheet having a tensile strength (hereinafter denoted as a T.S.) of not less than 80 kgf/mm.sup.2 and a yield ratio (hereinafter denoted as a Y.R.) of not more than 60%, which sheet is preferably used for members of an automobile, such as bumpers or bars for protecting the doors, which require high strength.
To reduce the weight primarily of automobiles, high-strength steel sheets are widely used as outer and structural materials for automobile bodies. Such steel sheets are required to have strength sufficient for meeting the demand of automobile safety, in addition to having excellent press workability.
In recent years, there has been an increasing demand for further reducing the weight of automobiles, as well as for protecting automobiles from rust. There has been a trend toward employing galvanized steel sheets for automobile members, including bumpers and bars for protecting automobile doors, whose weights have hitherto not been reduced.
As regards a type of galvanized steel sheet, having a T.S. of 80 kgf/mm.sup.2 or more, which is used for the members mentioned above, a galvanized steel sheet having a T.S. ranging from 100 to 120 kgf/mm.sup.2 is disclosed in Japanese Patent Laid-Open No. 1-198459. This sheet has yield strength ranging from 68.1 to 99.2 kgf/mm.sup.2, as high as 65% to 81% in terms of Y.R., thus resulting in a problem concerning form retention after having been worked.
As regards a type of cold-rolled steel sheet, a dual-phase type steel sheet of strength ranging from 100 to 120 kgf/mm.sup.2 is in use. Japanese Patent Publication No. 57-61819 discloses such a steel sheet employed as a plated steel sheet. This publication also discloses the fact that, when the dual-phase steel sheet is galvanized on a continuous galvanizing line having a low-temperature zone, the steel sheet transforms from .gamma. to .alpha. or from .gamma. to bainite. The amount of martensite is insufficient for obtaining strength ranging from 100 to 120 kgf/mm.sup.2.